1. Field of the Invention
The present invention relates to a power supply system provided with a nonaqueous electrolyte battery and a motor car provided with the power supply system.
2. Description of the Related Art
In recent years, vigorous research is being conducted on, for example, a hybrid vehicle, a motorcycle, an electric tram, an elevator and a windpower plant each having a battery mounted therein in order to utilize effectively the energy, and to utilize effectively the regenerative energy in view of the preservation of the earth's environment. Some of them have been in practical use. Also, secondary batteries have been studied as a backup power supply in case of emergency such as a power failure. Batteries that have been in practical use and mounted include a lead-acid storage battery and a nickel-hydrogen secondary battery.
However, for example, nickel-hydrogen secondary batteries used for hybrid vehicles have a problem that batteries suddenly produce heat during high power output or fast charging by regenerative energy, and also degradation by heat is severe. Also, lead-acid storage batteries used as an emergency power supply are subject to restrictions of installation locations because they are heavy due to small weight energy density.
On the other hand, hybrid vehicles using large-capacity capacitors have also been under development. However, while the capacitors can instantaneously store large electric power compared with the secondary batteries, there is a problem that the capacitors have a very small electric capacity and thus cannot be miniaturized.
In addition, for electric vehicles, a power supply system that can efficiently recover regenerative energy when a brake is applied has not yet been developed and thus, large-capacity batteries must be mounted.
Under such circumstances, high-power lithium-ion batteries have been developed to overcome the above problems. Lithium-ion batteries with high voltage and light weight have high energy density, but with the use of a carbonaceous material for a negative electrode, there is a problem of cycle life degradation caused, for example, by fast charging during energy regeneration or the like. Consequently, input power into the lithium-ion batteries must be limited and regenerative energy cannot be stored efficiently. Moreover, if output power of the lithium-ion batteries is made higher, discharge capacity will be decreased, making a travel distance of, for example, an electric vehicle (EV) or hybrid vehicle shorter. Also, for plug-in hybrid vehicles, making the travel distance by motor driving (EV running) longer will be demanded in addition to improvement of energy regeneration performance when a brake is applied and accelerating performance. However, it is difficult to have both improvement of energy regeneration performance and accelerating performance and improvement of traveling performance by motor driving (EV running) at the same time.
JP-A 2000-348725 (KOKAI) discloses that if carbon or lithium-titanium oxide is used as a negative electrode material, the potential changes little when the negative electrode is charged and thus an end point of charging must be determined by the positive electrode potential. As a result, fast charging performance will be deteriorated. Therefore, in JP-A 2000-348725 (KOKAI), an attempt is made to control the end point of charging through the negative electrode potential by forming a negative electrode active material from a carbonaceous material and a material (for example, lithium-titanium oxide) whose potential in a plateau area is higher than that of the carbonaceous material. In the plateau area, the potential does not change even if lithium is taken in.
However, lithium ion absorbing potential of the carbonaceous material is different from that of the lithium-titanium oxide. Therefore, if the carbonaceous material and the lithium-titanium oxide are used together in a cell, lithium ion absorbing/desorbing is unlikely to occur in the carbonaceous material, making it impossible to provide a high discharge capacity.